DE10117028B4 - Refractory Resistor, Method of Making, Offset and Shaped Articles Thereof - Google Patents

Abstract

From the melt solidified Resistor, particularly for a refractory mold body, or a refractory material is formed of at least one refractory mineral metal oxide main component, said elasticizer of the general formula A ²⁺ B ³⁺ ₂ O ₄ are present in such an amount that the solubility of the main component is exceeded and the elastifiers form precipitate fields in the main component, wherein the resistor is generated by co-melting of the main component and the elastifiers forming oxides and the elastifiers of the elements A ²⁺ = Fe, Mg, Mn, Zn and B ³⁺ = Fe, Al, Mn be formed.

Description

The The invention relates to a resistor for a refractory molding, a Process for its preparation, an offset for producing the refractory shaped body, a refractory molded body and a refractory composition, and a method of making the Molding.

As a resistor in the context of the invention, the carrier of the refractoriness and thus usually also referred to the main component of a refractory shaped body or refractory masses. This term has hitherto essentially included pure metal oxide, mineral, refractory substances such as MgO, Al ₂ O ₃ , doloma, ie calcined dolomite, spinel or the like.

When Elastifers are hereinafter referred to as minerals, due to their own, relatively high fire resistance, but one to the resistor different thermal expansion, by microcracking and further effects to an increase the thermal shock resistance a mixture of Resistor and Elastifizierer - in contrast to the pure resistor - lead.

refractory Moldings, in particular basic refractory materials based on Magnesia and Doloma are involved in all high-temperature processes basic slag attack, e.g. in cement, lime, dolomite, Iron and steel production and non-ferrous metal production and in the glass industry, as lining material for stoves and Used vessels. High fire resistance and good chemical resistance However, these materials or moldings have a high brittleness, i.e. a high modulus of elasticity on.

in this connection It should be noted that moldings on Base of fused magnesia are significantly more brittle than molded bodies the base of sintered magnesia. Naturally, however, the enamel magnesia has a much higher one Resistance to thermochemical attack on as sintered magnesia. In that sense would be it desirable, fused magnesia or shaped body or to use masses based on fused magnesia in areas in those a high thermochemical attack, especially an attack thinner, basic slag, takes place. This is especially true in the rotary kiln for the Cement production the case. Especially in cement rotary kilns is However, a significant mechanical load on the refractory lining present, so that known moldings based on fused magnesia in such firing aggregates not can be used successfully because of their brittleness the entered voltages, in particular ring voltages, not coped.

Shaped on The basis of fused magnesia are molded bodies based on sintered magnesia inferior in such burners, if an elastification of the molding he follows.

to Elastification, i. to improve the thermal shock resistance (TWB) basic, refractory materials have been used in the past numerous measures taken. For example, in Harders / Kienow, refractories, production, Properties and use of refractory materials, Springer-Verlag 1960, chapter 5.5, page 755, proposed basic, refractory Materials with chrome ore, in particular the chromium ore and the optimum grain fraction of the chrome ore were defined. Around to obtain a sufficient thermal shock resistance, are while chromium ore amounts between 15 and 30 wt .-% necessary. The elasticizing, i.e. the E-modulus lowering effect of the chromium ore as a thermal shock resistance component is by W. Späth in "For thermal shock resistance refractory materials ", Radex Rundschau, vintages 1960-1961, page 673-688, Austrian-American Magnesitaktiengesellschaft, Radenthein / Carinthia, by structural tension due to different thermal expansion coefficients between Magnesia and Chrome ore. Decisive disadvantages the application of chrome ore as a thermal shock resistance improver are, however, that at a change of the furnace atmosphere material fatigue take place and that through Oxidation under the action of alkalis that in chromium ore in trivalent Form chromium oxide is converted into toxic, hexavalent chromium oxide, with all thus under occupational hygiene and disposal technology Aspects related problems.

It is also known from AT-PS 158 208, Magnesiasteinen to improve the thermal shock resistance alumina powder, corundum and aluminum powder to add, wherein the rock fire in situ spinel is formed. The resulting aluminum-magnesium spinel is concentrated in the matrix and partly not fully reacted, so that when attacking such stones by slag preferably decided on the strength dende matrix is destroyed. Such a magnesium-aluminum spinel also has a different coefficient of thermal expansion compared with pure magnesia, so that this also results in structural stresses and thus in microcracks.

A Significant improvement in both the thermal shock resistance as well as the chemical resistance of magnesia stones could only be achieved by adding pre-synthesized Magnesium-aluminum spinel in the form of sintered or melted spinel be achieved, the usual Additional amounts between 15 and 25 wt .-% are. The modulus of elasticity let yourself with this measure lower to about 20 GPa. However, there are still disadvantages that these Spinel component has a reactivity with slags and thus a wear in the The area of the spinel matrix takes place, which ultimately also becomes one accelerated Resistor degradation leads.

From the DE 35 27 789 A1 is a coarse-ceramic molded body is known, which has a substantially homogeneously distributed in the molding structure micro-crack system. This document is based on the finding that a low modulus of elasticity with high resistance to slag attack is produced by the fact that the microstructure homogenous microcracking far greater diameter than in the case of, for example, dense, oxide-ceramic high-temperature materials distributed, the Mechanism is based either that this during the firing process or stone sintering an expansion of the respective particles, ie a volume-increasing reaction triggers, in the adjacent, other particles then the micro-cracking system, or a strong shrinkage in the flour fraction is caused, in turn leads to the described microcracking system in the other particles of the batch. For this purpose, pure magnesia and alumina are mixed in a stoichiometric ratio corresponding to the magnesium aluminate spinel and shaped into aggregate particles, which are then added to the base mixture of sintered magnesia. Such refractory moldings have proven themselves. In particular, when used in highly mechanically stressed units with high basic slag attack, such as rotary kilns in the cement industry, however, it comes with such moldings to rapid wear.

• – 23 bis 55 Gew.-% FeO
• – < 15 Gew.-% MgO
• – 54 bis 65 Gew.-% Al₂O₃
• – < 3 Gew.-% Verunreinigung

From the DE 44 03 869 A1 is a refractory, ceramic mass and their use is known, said refractory, ceramic mass of 50 to 97 wt .-% MgO sinter and 3 to 50 wt .-% of a spinel of the hercynite type is to exist. In this document it is stated that, for example, for the lining of industrial furnaces, in which a significant mechanical stress of the refractory lining occurs, products are required whose brittleness is as low as possible. These furnaces would include rotary kilns in the cement industry, where furnace deformation could result in significant mechanical loading of the refractory lining, but furnaces of the steel and nonferrous metals industry would also be involved, especially where thermal stresses during heating and temperature cycling would cause problems , In contrast to chromium-containing stones, it is proposed to use hercynite or a hercynite-like spinel as the elasticizer, the hercynike spinel being within the following range limits of the ternary state diagram FeO - Al ₂ O ₃ - MgO:

• - 23 to 55 wt .-% FeO
• - <15 wt .-% MgO
• - 54 to 65 wt .-% Al ₂ O ₃
• - <3 wt .-% impurity

The refractory bricks made and fired using this spinel are said to have significantly improved ductility. Furthermore, it is stated in this document that the sintered magnesia could also be replaced by fused magnesia. However, such refractory products have the disadvantage that the hercynite-type spinels tend to dissolve and take up MgO from the refractory resistor. The solubility limit of the hercynite-type spinel for MgO is 15 to 20%. Conversely, the MgO is able to incorporate parts of the hercynite spinel or its constituent oxides FeO and Al ₂ O ₃ . It was therefore observed in such refractory moldings an undesirable Gefügeschwächung by diffusion processes and mutual dissolution of the constituents, with a significant diffusion gradient in the direction of MgO to spinel because of the higher diffusion rate of Mg ²⁺ is (W. Schulle, refractory materials, German publishing house for basic industries , 1990, p. 254). It is also disadvantageous that, when using fused magnesia, the elasticizing portion of the spinel is not high enough to make this resistor suitable for rotary kilns with sufficiently high thermochemical resistance.

In the German patent DE 198 59 372 C1 discloses a basic refractory ceramic composition based on sintered magnesia. To increase the ductility are produced from Sintermagnesia Stei Galaxel type (Mg, Mn) (Fe, Al) ₂ O ₄ or Jacobsite type (Mg, Mn) (Fe, Al) ₂ O ₄ spinels are added in the mixture. Stones made of this material are sintered in such a way that complete, dense sintering between the individual mass constituents does not take place, so that the fired, refractory molded body produced from the mass also retains a certain flexibility.

The German Auslegeschrift DE 1 771 6721 B describes melt-molded bodies whose main constituent is magnesia, the rock containing chromium. The metallic ferrochrome used before a melting process is present in the final product both dissolved in the periclase and as spinel precipitate. The achieved heat resistance properties are mainly attributed to the lack of a significant Al ₂ O ₃ content. Stones produced by the process presented have a better thermal shock resistance than conventional magnesia-chromite fused-material bricks.

In the German patent DE 15 71 392 C3 describes a temperature change resistant, Schmelzggossener fire resistant Magnesia-Chromitgusskörper with 40 to 78 wt .-% MgO, 10 to 55 wt .-% Cr ₂ O ₃ and 4 to 30 wt .-% Al ₂ O ₃ , intragranular grains of a chromium-containing Spinells within Periklaskörnern and intergranular, chromium-containing spinel grains between these Periklaskörnern present. Shaped bodies of this material have a lower tendency to break off excesses on the hot surface of the stones, thereby prolonging the life of the stones. The good corrosion resistance of the described castings compared to basic slag is attributed to the very small proportion of microcracked periklaskörnern. Low intergranular spinel content reduces microcracking. In order to prevent the formation of a thermally unstable microstructure containing large amounts of intergranular spinel and in particular idiomorphic-hypidiomorphic intergranular spinel in the manufacture of a cast body, the molten and cast magnesia-chromite casting material is allowed to solidify at a high speed to produce a special one Microstructure forms. Higher solidification rates were found to suppress the formation of intergranular spinel and favor the formation of intragranular spinel.

task The invention is to provide a refractory resistor, the with a very high thermochemical resistance sufficient ductility also for has the use in mechanically highly stressed aggregates, and a method for its production, an offset and a moldings it.

These The object is solved by the features of claims 1, 14, 23, 31 and 33.

advantageous Embodiments are given in the respective dependent subclaims.

According to the invention is a Melt magnesia or generally a molten and thus inherently brittle resistor conditioned so that he an increased ductility or higher elasticity so that this Resistor with high thermochemical resistance also in mechanical highly loaded units such as rotary kilns can be used. This is inventively characterized achieved that the Spinel former during the melting process is present, so that the resulting after melting Resistor grains excreta or precipitation fields in which spinel concentrates is. In this case, the addition of spinel formers is metered in this way, that the solubility of the resistor for exceeded this spinel former is, whereby by the exact metering of the spinel former thus the number and, surprisingly, too the size of the elimination fields is determinable.

It is also surprising that one sufficient elasticity of the molten resistor is possible, although between the individual, while the sintered firing interconnected fused magnesia or Schmelzresistorkörnern hereby no visible microcracking system is produced.

From Another advantage of such a conditioned resistor is that the elasticizer, which concerning the chemical resistance represents the weak point of the stone, from more chemically resistant Resistor shielded against the slags becomes. Here you can with the addition amounts according to the invention from spinel formers even such high elastification performances be achieved that the Resistor is conditioned so far that it next to its "internal" Elastifizierer (elimination fields) no further external elastifier additions needed in the offset.

Of course it is it still possible add further elastifiers (external elastifiers).

It it turned out that it It is particularly advantageous to use a spinel as an external elasticizer as it also works as an internal elasticizer. This is attributed to the fact that the resistor is so far saturated with the spinel by the self-elasticity, that one Diffusion inhibition or reduction is generated thereby. The influences by diffusion, for example, a pure hercynite, on the resistor are thereby prevented.

In the main field of application according to the invention, namely fused magnesia, FeO and Al ₂ O ₃ or Fe ₂ O ₃ and Al ₂ O ₃ are preferably added so that the precipitation fields are formed essentially of a Pleonasts spinel or a spinel of the Pleonast type. It has been found that this Pleonast type of elasticizer, both internally and externally, has significantly improved compatibility with the resistor, with sufficient elasticity ensured. Furthermore, this elasticizer has increased thermochemical resistance over known elastomers.

The Invention will be exemplified below with reference to a drawing explained. It shows here

1 an inventive melting grain of MgO with pleonastic, punctiform demixing in the grain and at the grain boundaries;

2 a further recording of a melting grain of MgO invention with point-like aluminum spinel segregations in the grain and at the grain boundaries.

3 the composition range of a periclas-based resistor according to the invention conditioned in the ternary system FeO _x -Al ₂ O ₃ -MgO,

4 the process tree of the preparation of the inventive resistor and molded body thereof.

According to the invention, a resistor, ie the carrier of the refractoriness of a refractory batch, usually comprises a metal oxide, mineral, granular component, demixing fields of spinel. In particular, a resistor based on MgO is selected according to the invention, this resistor consisting of a doped magnesium oxide which contains denaturations of spinel. The spinel itself may have a composition corresponding to (Fe, Mg, Mn, Zn) ²⁺ (Fe, Al, Mn) ³⁺ ₂ O ₄ . The amount of these spinel mixtures in the magnesia or the refractory resistor can be between 2 and 25 wt .-%. Advantageously, this spinel mixture is used in molten magnesium oxide, the so-called. Schmelzmagnesia.

in the Shaped body according to the invention less corrosion possible, since the spinel is incorporated in the MgO grain and the microstructure thereby its elasticity keeps permanently. In contrast, in the prior art remains after the wear of the Elastifizierers a scaffolding out of a brittle one Material which, after the elasticizer has worn, accelerates further wear can. Also a local accumulation of corrosion products, such as with moldings occurs in which the elastifiers in granular or locally highly enriched Form are avoided.

Like the two pictures according to 1 and 2 show, the spinel mixtures are distributed in a punctiform manner throughout the enamel grain relatively uniformly, of course, spinel mixtures are also present in the grain boundary regions. However, if these are corroded, they represent only a small proportion of the spinel mixtures actually present, so that the elasticizing effect of the spinel mixtures is only minimally impaired. It is therefore possible to deliver thermomechanically sensitive aggregates with a shaped body containing the invention elasticized fused magnesia, which compared to the prior art, a reduced thermo-mechanical sensitivity, since even the grain has improved elasticity and plasticity and also the corrosion resistance is improved , These effects can be determined on the one hand via the Ela stizitätsmodul, as a measure of the elasticity, and on the other about D _max from the measurement of the pressure softening according to DIN 51053 (with a mechanical load of 0.2 MPa), as a measure of the hoop stress or the plastic deformation, prove, since at high D _max also higher tensions build up, which lead to a chipping of stone layers and thus to premature destruction of the refractory lining. At a low D _max , the mechanical stresses can already degrade without destruction due to plastic processes.

In the pictures according to 1 and 2 are the big, white fields 1 Periclase crystals, which in the range of indicated cracks or grain boundaries 4 abut. In the Periklaskristallen are the punctiform Spinellentmischungen 3 to recognize, with voids or pores 2 available. The punctiform separations 3 in 1 are pleonastic spinels, while the punctiform segregations 3 in 2 Magnesium aluminum spinels are.

Basically Such a conditioned fused magnesia also together with conventional sintered magnesia used if this is desired for certain reasons, such as cost reasons.

When Starting materials are especially caustic magnesia, magnesium hydroxide and magnesite used to form the spinel mixtures Alumina, for example in the form of tabular earth, and iron oxide, for example in the form of magnetite.

Of course you can to form spinel mixtures also only alumina be added to the formation of aluminum-magnesium spinel. Furthermore, it goes without saying possible, all Spinel-forming minerals corresponding to a stoichiometrically required Add amount as spinel former, i. the corresponding oxides the elements iron, magnesium, manganese and aluminum or more potential Spinel.

Thus, for example, in a fused magnesia, the corresponding oxides of iron, manganese and aluminum may be added. In the case of a resistor based on Al ₂ O ₃ , the oxides of iron, manganese and magnesium could be added accordingly.

In 3 exemplifies the field of a self-elasticized periclasis according to the invention with a hatched field 1 shown.

in the Following, the invention will be explained with reference to embodiments.

Example 1:

90 wt .-% of a caustic magnesia, 4.4 wt .-% iron oxide and 5.6 wt .-% alumina are melted in an electric arc furnace at a temperature of about 3,000 ° C ( 4 ). After the melting process and the subsequent cooling, the molten product is processed into fractions up to 1 mm, 1 to 2 mm, 2 to 4 mm and flour. The offset composition for the production of the refractory shaped body is carried out in accordance with a typical Fullerkurve. The granulation achieved by the composition of the fractions is mixed with a required amount of lignosulfonate and pressed with a specific compression pressure of 130 MPa to give moldings. After drying, the stone is fired at a sintering temperature of about 1,600 ° C. For comparison, the same offset is made with a magnesia which is also melted but which contains no additional iron oxide and no additional alumina. After the stone fire, the elastic property and the thermal-mechanical quantity D _{max are} measured on this stone in order to be able to make a statement about the elastic and plastic behavior.

The achieved properties are listed in the following table: Table 1

It can be seen from this table that the values of the elastic properties of a magnesia brick formed from a resistor with pleonastent mixtures are far below the typical values of pure magnesia stones. At the same time, due to the plastic, crack-free stress relief, the D _max value is surprisingly lowered by more than 20%, as a result of which the mechanical stresses which result in an annular aggregate or when the lining is clamped in a mechanical frame, for example in the case of a stationary furnace, also decrease significantly.

Example 2:

85 wt .-% magnesia and 15 wt .-% alumina are melted in an electric arc furnace at a temperature of about 3,000 ° C ( 4 ). The melt product contains demixings of magnesium-aluminum spinel, as in 2 are shown. This material is processed after the melting process in fractions up to 1 mm, 1 to 2 mm, 2 to 4 mm and flour. The offset composition for the production of a refractory shaped body is carried out according to a typical Fullerkurve. The granulation composed of the individual fractions corresponding to the fuller curve is mixed with a per se customary and required amount of lignin sulfonate as a temporary binder and pressed with a specific compacting pressure of 130 MPa. After drying, the molded body is fired at a sintering temperature of 1,600 ° C. For comparison, a molded body of pure magnesia, as in Example 1, used. The measured quantities correspond to those of Example 1. The properties achieved are listed in the following table: Table 2

It can also be seen from this table that the values of the elastic properties of the spinelene-containing resistor are far below the typical values of pure magnesia stones. At the same time, due to the plastic, crack-free stress reduction, the D _max value is also lowered by more than 20%, as a result of which the mechanical stresses in refractory masonry of an industrial furnace also drop significantly.

Produced according to the invention moldings can be everywhere Use where strong mechanical and thermo-mechanical stresses occur. Across from previously known moldings with the natural brittle MgO grain, in particular melted grain, has the inventive resistor an increased plasticity and elasticity. in this connection is an advantage that the elasticity and the plasticity in accordance with the invention Resistors in the grain itself is generated.

Of course, the use of the inventively produced resistor does not exclude an application in refractory moldings containing other elastifiers such as spinel, hercynite, zirconium oxide or chrome ore. Here, the positive effects of the self-relieved resistors and the added elastifiers can add together, resulting in a further improved elasticity and high-temperature plasticity. This can be seen in the following table: Table 3

Furthermore it goes without saying also possible, produced according to the invention Resistors along with other usual ones Resistors to be used in refractory masses or moldings.

Claims (35)

From the melt solidified Resistor, particularly for a refractory mold body, or a refractory material is formed of at least one refractory mineral metal oxide main component, said elasticizer of the general formula A ²⁺ B ³⁺ ₂ O ₄ are present in such an amount that the solubility of the main component is exceeded and the elastifiers form precipitate fields in the main component, wherein the resistor is generated by co-melting of the main component and the elastifiers forming oxides and the elastifiers of the elements A ²⁺ = Fe, Mg, Mn, Zn and B ³⁺ = Fe, Al, Mn be formed. Resistor according to Claim 1, characterized that the Main component contains MgO. Resistor according to Claim 1 or 2, characterized in that the main component contains Al ₂ O ₃ . Resistor according to one of the preceding claims, characterized characterized in that Main component Doloma contains. Resistor according to Claim 1 or 2, characterized that the Main component periclase is. Resistor according to one of Claims 1 to 4, characterized that the Main component is corundum. Resistor according to one of the preceding claims, characterized in that the elasticizer has a pleonastic spinel of the general formula (Fe, Mg) Al ₂ O ₄ . Resistor according to one of the preceding claims, characterized in that the elastifier ei has a MgAl spinel. Resistor according to one of the preceding claims, characterized in that the elasticiser is a pleonastic spinel of the general formula (Fe, Mg) Al ₂ O ₄ . Resistor according to Claim 7, characterized that the Elastolyzer is MgAl spinel. Resistor according to one of the preceding claims, characterized characterized in that Resistor 2 to 25 wt .-% of precipitates of the Elastifizierers. Resistor according to one of the preceding claims, characterized characterized in that Resistor 70 to 98 wt .-% of the refractory mineral metalloxidischen Main component has. Resistor according to one of the preceding claims, characterized characterized in that Resistor of caustic magnesia, magnesium hydroxide or magnesite and iron compounds, especially iron oxides such as magnetite, and Alumina is melted. Method for producing a refractory resistor according to one of the claims 1 to 13, characterized in that a refractory, metal oxide Main component and spinel-forming oxides mixed together and melted together, with the amount of spinel-forming Oxides is dimensioned such that the solubility the refractory mineral metal oxide main component for these spinel-forming oxides exceeded so that the spinel-forming oxides on cooling the melt spinel precipitates in the refractory mineral form metal oxide main component and as spinel forming Mineral oxides of the elements Fe, Mg, Mn, Zn, Al are used. Process according to Claim 14, characterized in that MgO and / or Al ₂ O ₃ and / or Doloma are used as refractory mineral metal oxide main component. Method according to claim 14 or 15, characterized that as refractory mineral metal oxide main component periclase is used. Process according to one of claims 14 to 16, characterized in that metal oxides are used as spinel formers which form a MgO-saturated pleonastic spinel of the general formula (Fe, Mg) Al ₂ O ₄ in the resistor. Method according to one of claims 14 to 17, characterized that the Amount of added metal oxides is adjusted so that the resistor Contains 2 to 25 wt .-% of precipitates of the spinel. Method according to one of claims 14 to 18, characterized in that so much mineral metal oxide main component is added, that the resistor 70 to 98 wt .-% of refractory mineral metal oxide Main component contains. Method according to one of claims 14 to 19, characterized that as Main component of caustic magnesia, magnesium hydroxide or magnesite and as spinel former iron compounds, in particular iron oxides, such as magnetite, and alumina are used. Method according to one of claims 14 to 20, characterized that the caustic magnesia, the magnesium hydroxide or magnesite and the Iron compounds, in particular iron oxides, such as magnetite, and aluminum oxide, like alumina, in an electric arc furnace at a temperature of> 2,500 ° C melted become. Method according to one of claims 14 to 21, characterized that after the melting process and subsequent cooling the molten product in fractions up to 1mm, 1 to 2mm, 2 to 4mm and flour is processed. Offset for the production of refractory moldings or refractory masses, characterized in that it comprises a resistor according to a the claims 1 to 13 contains in particular produced by a process according to one or more the claims 14 to 22. Offset according to claim 23, characterized that the Resistor in fractions up to 1mm, 1 to 2mm, 2 to 4mm and flour is present. Offset according to claim 23 or 24, characterized that the Offset for the production of refractory moldings or refractory masses according to a typical fuller curve, so that one granulation through the composition of the political groups and, if necessary, further additions in accordance with the Fullerkurve is achieved. Offset according to one of Claims 23 to 25, characterized that the Offset with at least one further elasticizer, such as MgAl spinel, Hercynite, zirconium oxide is added. Offset according to claim 26, characterized that the additional elastifiers to the elasticizer in the precipitation fields equivalent. Offset according to one or more of claims 23 to 27, characterized in that is offset with a further resistor. Offset according to claim 28, characterized that he is added with sintered magnesia. Offset according to one of Claims 24 to 27, characterized that he with a temporary binder, in particular with ligninsulfanate is added. Refractory body or refractory mass comprising a resistor as a carrier the refractoriness, characterized in that the resistor is a resistor according to one or more of the claims 1 to 13, in particular produced by a method according to one or more of the claims 14 to 22, in particular from an offset to one or more the claims 23 to 30. moldings or composition according to claim 31, characterized in that the shaped body or the mass at> 1000 ° C, in particular between 1200 and 1750 ° C is burned. Process for the preparation of a refractory molding according to Claim 31 or 32 from an offset according to one or more of claims 23 to 30, characterized in that the offset with a required Amount added to binder and with a specific pressure of> 50 MPa, in particular 80 to 200 MPa, preferably 100 to 150 MPa, is pressed into shaped bodies. Method according to claim 33, characterized that the dried green moldings becomes. A method according to claim 33 and / or 34, characterized characterized in that dried moldings at> 1000 ° C, in particular between 1,200 and 1,750 ° C, is burned.